1. Field of the Invention
This invention relates to an electrode catalyst for a fuel cell and a method for the production thereof, and more particularly to an electrode catalyst for a fuel cell possessing a core-shell structure comprising a core part formed of a noble metal alloy and a shell part formed of a noble metal-containing part different in composition from the core part and formed on the periphery of the core part and a method for the production of the electrode catalyst.
2. Description of the Related Art
The hydrogen-oxygen fuel cells are classified based on the kind of electrolyte and the kind of electrode into varying types, which are represented by alkali, phosphoric acid, fused carbonate, solid electrolyte, and solid polymer fuel cell.
As a cathode catalyst for a solid polymer fuel cell or a phosphoric acid fuel cell, for example, a catalyst which has a noble metal inclusive of platinum deposited on carbon black is used. The platinum-deposited carbon black is produced by adding sodium hydrogen sulfite to an aqueous solution of platinum chloride, mixing the resultant with aqueous hydrogen peroxide solution, depositing the consequently educed platinum colloid on carbon black, washing obtained composite, and optionally heat-treating the washed composite. Among them, the feasibility of the solid polymer fuel cell as the power source for an automobile or a stationary device is being tried. Since this fuel cell is required to retain high durability and necessary ability to generate power for a long time, it has been continued various improvements.
The platinum alloy catalyst for use in the phosphoric acid fuel cell, for example, entailed the problem that the platinum alloy catalyst was alloyed only to an approximate degree of 1:0.3, the base metal as a component of the alloy was educed as crystals of a salt, and the base metal component was maldistributed in the catalyst. Then, a method has been proposed which comprises alloying platinum or a platinum alloy in a catalyst by depositing a base metal on the particles of the platinum or platinum alloy thereby decreasing a platinum solo particle or a platinum alloy solo particle and approximating the alloy composition ratio closely to 1:1 as well, and decreasing uneven distribution of the alloy composition ratio in the catalyst and enhancing the liquation resistance and the catalyst activity (JP-A-1994-7679). In the JP-A-1994-7679, a catalyst having platinum or a platinum alloy deposited on carbon powder is dispersed with a supersonic homogenizer while keeping the catalyst immersed and stirred in a hot water, and thereafter electroless plating the dispersed catalyst with one or more members of chromium, manganese, iron, cobalt, nickel, and copper thereby alloying them with the platinum or the platinum alloy in the catalyst. To solve the same problem, another method has been proposed which comprises dispersing a noble metal or noble metal alloy catalyst deposited on a carrier in a hot water thereby preparing a catalyst slurry, introducing a heavy metal-containing electroless plating liquid in the catalyst slurry and executing electroless plating of the slurry therein, and heating the deposited noble metal or noble metal alloy catalyst at a temperature in the range of 850–950° C. thereby alloying the catalyst (JP-A-1996-141400).
Under the conditions for using a fuel cell that requires coexistence of an acid electrolyte and oxygen, the gradual liquation into the electrolyte of not only the base metal not alloyed with platinum but also the base metal in the crystals of the alloy of the base metal and platinum is inevitable. For solving the liquation, a technique has been proposed which comprises mixing a specific base metal and platinum thereby composing an alloy possessing a vacancy type lattice defect structure and depositing this alloy on an electroconductive carbon powder (JP-A-1998-69914). An obtained skeleton catalyst which is produced by selectively removing a base metal component from the crystals of an ally of a specific base metal and platinum, when used as an electrode catalyst for a fuel cell, exhibits high activity and lasting stability.
Further, when such a reformed gas as hydrocarbon or methanol is used as the fuel for a fuel cell, for example, the fuel includes some tens of ppm of carbon monoxide besides hydrogen and carbon dioxide and this carbon monoxide poisons the platinum catalyst of the anode. To preclude such a deterioration of a catalyst, a catalyst that results from alloying ruthenium and platinum is used as a catalyst for the anode. Actually, when this catalyst is used under the operating conditions of an electrolyte type fuel cell, the catalyst shows a slight sign of growth of platinum particles during the use. This phenomenon may be ascribed to the growth of particles due to the mechanism of fusion and precipitation of platinum and to the mutual fusion of platinum particles which is induced by the breakage of junctions between platinum and a carbon material. The growth of particles in the platinum catalyst causes decline of the characteristic properties when the amount of platinum is small. To repress the growth an electrode catalyst for a fuel cell, a particle of a metal which is less oxidizable than platinum under an acidic state is deposited on an electroconductive material and the outer surface of the particle is coated with platinum (JP-A-2002-289208).